Results 1 - 10 of 11652
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[en] In this study, fluorite structured oxides are used in numerous applications and as such it is necessary to determine their materials properties over a range of conditions. In the present study we employ molecular dynamics calculations to calculate the elastic and expansivity data, which are then used in a thermodynamic model (the cBΩ model) to calculate the activation volumes of oxygen self-diffusion coefficient in ThO_2, UO_2 and PuO_2 fluorite structured oxides over a wide temperature range. We present relations to calculate the activation volumes of oxygen self-diffusion coefficient in ThO_2, UO_2 and PuO_2 for a wide range of temperature (300–1700 K) and pressure (–7.5 to 7.5 GPa).
[en] Thin films of magnetic garnet materials, e.g. yttrium iron garnet (Y3Fe5O12, YIG), are useful for a variety of applications including microwave integrated circuits and spintronics. Moreover, substitution of rare earth ions, such as cerium, is known to enhance the magneto-optic Kerr effect (MOKE) as compared to pure YIG. Thin films of Ce0.75Y2.25Fe5O12 (Ce:YIG) have been grown using the pulsed laser deposition (PLD) technique and their crystal structure examined using high resolution scanning transmission electron microscopy. Homogeneous substitution of Ce in YIG, without oxidation to form a separate CeO2 phase, can be realized in a narrow process window with resulting enhancement of the MOKE signal. We investigated the thermally generated signal due to spin Seebeck effect for the optimally doped Ce:YIG films.
[en] Ge1−xSnx alloy thin films were prepared by co-sputtering from Ge and Sn targets on a Si (100) substrate at room temperature, and were then heated at temperature ranging from 200 to 500 in N2 ambient to reduce the disorder and defects and increase the crystalline quality of the films. Images obtained by field emission scanning electron microscopy revealed that the as-grown and all annealed samples displayed a densely packed morphology. The atomic percent composition of Sn in the as-grown Ge1−xSnx film is 5.7 at. Energy-dispersive x-ray spectroscopy results showed Sn surface segregation after heat treatment, as the Sn composition is reduced to 3.3 at for the film annealed at 500 C. The Raman analysis showed that the only observed phonon mode is attributed to Ge–Ge vibrations. The Raman spectra of as-sputtered and annealed films revealed their nanocrystalline-amorphous nature. The samples annealed at lower temperature exhibited higher phonon intensity, indicating the improvement of crystallinity of the film. The optoelectronic characteristics of fabricated metal-semiconductor-metal photodetectors on the annealed sample at 200 and the as-sputtered sample were studied in the dark and under illumination. Compared with the as-sputtered one, the annealed sample showed lower dark current and higher current gain of 209. The results showed the potentiality of using the sputtering technique to produce GeSn layer for optoelectronics application. (paper)
[en] Single-phase, near-stoichiometric zinc stannate (Zn2SnO4) nanowires were synthesized by a direct vapor transport process on c-Al2O3 substrates under optimized growth conditions. The optimal growth temperature for Zn2SnO4 nanowires is above 700 °C. Structural characterization indicates that the nanowires had the single crystal cubic spinel structure and diameters of ∼90 nm and they grew in the [1 direction. Low-temperature photoluminescence (PL) shows a strong emission peak at 3.7 eV, attributed to the near-band-edge-emission of Zn2SnO4 nanowires. Temperature-dependent PL results are consistent with the Varshni equation, and the band gap is redshifted by ∼170 meV as the temperature is increased from 11 to 300 K. The obtained direct gap of Zn2SnO4 nanowires is 3.546 eV at 300 K. A UV photodetector based on as-grown Zn2SnO4 nanowires was fabricated by a simple and cost-effective process. The Zn2SnO4 nanowires exhibited UV photoconductivity, and good selectivity and decent response to UV. The efficient fabrication method, high chemical and thermal stability of the Zn2SnO4 nanowire UV photodetector made it very suitable for use in harsh environments. (paper)
[en] An understanding of the microscopic processes underlying the magnetization of the thin films is essential to the design and development of efficient magnetic recording media. In this context, we measure the magnetic Compton profile (MCP) of TbxCo100−x films (13 x 20) and investigate the microscopic (spin magnetic moment (), orbital magnetic moment (), and element specific magnetic moment ( and )) magnetization processes. Using the magnetic-field dependence of MCP, we obtain the spin- and orbital-specific magnetization curves (SSMH and OSMH, respectively). From the profile-fitting analysis, we also obtain the element specific SSMH curves, and we find from our results that the ratio is nearly constant. Thus, it is assumed that the magnetic switching processes of and cooperatively occur in this system (corresponding to and ). In addition, we observe that and are −1 at x 17. This implies that the magnetic moment of Tb aligns toward the magnetic field in the region of x > 17 and that of Co aligns toward the magnetic field in the region of x < 17. We estimate that this value of x 17 is the compositional point that corresponds to the compensation composition of Tb and Co. (paper)
[en] First principles investigations are performed to study the half-Heusler compounds CrScZ (Z P, As, Sb). In our calculations the electronic exchange correlation energy is determined by two approaches, generalized gradient approximation (GGA) and generalized gradient approximation with additional Hubbard-U term for indulging on-site Coulomb interaction (GGA+U), independently. Equilibrium lattice constants are found to be in the range of 5.35–6.60 Å. It is revealed by the analysis of elastic properties that two of the compounds, CrScP and CrScAs, are ductile while the third compound, CrScSb, is brittle in nature in the GGA approach, but all the three compounds are predicted to be ductile in the GGA+U approach. The results of both calculations are shown for comparison. Half-metallicity of these compounds is verified, using both methods, by showing that the spin-down states are semiconducting while the spin-up states are conducting. These compounds possess relatively high total magnetic moments of about ≈. Several optical properties are also calculated that include the refractive index, extinction coefficient, reflectivity, conductivity, and absorption coefficient. (paper)
[en] A series of polycrystalline samples of Y3 M (M = Co, Ni, Rh, Pd, Ir, Pt), intermetallic binary compounds were synthesized by the arc-melting method. Powder x-ray diffraction (pXRD) confirmed the orthorhombic cementite-type crystal structure and allowed for the estimation of the lattice parameters. Physical properties were investigated by means of electrical resistivity and heat capacity measurements between 1.9 K and 300 K. All tested compounds show metallic-like behaviour with RRR values ranging from 1.3 to 8.3, and power-law temperature dependence of resistivity was observed, with . No superconductivity was detected above 1.9 K. The Debye temperature, estimated from the low temperature heat capacity fit, ranged from 180 K (Y3Pt) to 222 K (Y3Co). The highest value of the Sommerfeld coefficient γ was found for Y3Pd (19.5 mJ mol−1 K−2). The pXRD pattern of Y3Rh indicated the presence of Y5Rh2, a previously unreported Pd5B2-type phase, whose unit cell parameters were refined using the LeBail method. Density functional theory calculations were performed and theoretical results revealed strong enhancement of the measured electronic specific heat, which was 30%–100% larger than computed. Quadratic temperature dependence of resistivity and enhanced electronic specific heat indicated a Fermi-liquid behavior of electrons in these materials. (paper)
[en] We examine high quality, single crystal CdTe epilayer grown by molecular beam epitaxy (MBE) on ()B GaAs substrate using both positions and full width at half maximums (FWHMs) of reciprocal lattice points (RLPs). Our results demonstrate that reciprocal space mapping (RSM) is an effective way to study the structural characteristics of the high-index oriented epitaxial thin films having a large lattice mismatch with the substrate. The measurement method is defined first, and then the influence of shear strain () on the position of the () node of epilayer is clarified. It is concluded that the lattice tilting is likely to be related with the lattice mismatch. Nondestructive measurement of the dislocation density is achieved by applying the mosaic crystal model. The screw dislocation density, estimated to be cm−2, was calculated utilizing the broadened peakwidths of the asymmetric RLP of the epilayer lattice. (paper)
[en] Carbon nanotubes (CNTs) have great potential for use as electrical wires because of their outstanding electrical and mechanical properties. Here, we fabricate lightweight CNT fibers with electrical conductivity as high as that of stainless steel from macroscopic CNT films by drawing them through diamond wire-drawing dies. The entangled CNT bundles are straightened by suffering tension, which improves the alignment of the fibers. The loose fibers are squeezed by the diamond wire-drawing dies, which reduces the intertube space and contact resistance. The CNT fibers prepared by drawing have an electrical conductivity as high as 1.6 × 10"6 s m"−"1. The fibers are very stable when kept in the air and under cyclic tensile test. A prototype of CNT motor is demonstrated by replacing the copper wires with the CNT fibers. (paper)
[en] Electron emission properties of electrodeposited spinel NiCo_2O_4 nanosheet arrays grown on Ni foam have been studied. The work function of NiCo_2O_4 was calculated by density functional theory using the plane-wave basis set and used to estimate the field enhancement factor. The NiCo_2O_4 nanosheet arrays exhibited a low turn-on field of 1.86 V μm"−"1 at 1 μA cm"−"2 and current density of 686 μA cm"−"2 at 3.2 V μm"−"1, with field enhancement factor β = 1460 and good field emission current stability. The field emission properties of the NiCo_2O_4 nanosheet arrays showed enhanced performance compared to chemically prepared NiCo_2O_4 nanosheets. Hence, the nanosheet arrays have great potential as robust high performance vertical structure electron emitters for future flat panel displays and vacuum electronic device applications. (paper)